Is this canning method safe to use for spaghetti sauce? (Boiling the sauce in the jars in the oven)

No, it is not safe. You need a pressure canner. That's what the USDA says about anything containing meat:

There are no safe options for canning these foods listed below in a boiling water canner.

See http://nchfp.uga.edu/how/can_04/soups.html for a table of pressures and processing times.

In general (if it is about random sauce):

Unless the sauce is so heavy in sugar (unlikely - 120°C would mean you are making a tomato syrup that will be as thick as honey when cool), oil or thickeners that it will reach pressure-canning temperatures when heated by an oven - NO.

The cans are at ambient pressure, so any mixture in them that is dominated by water WILL NOT reach much above a 100°C, since heat energy will be used up for boiling off water and not raising the temperature of the sauce.

100°C will kill any practical live bacteria to hell and back, but it will not kill spores.

Fully closing the cans would get you higher internal temperature - DON'T, they will likely explode very violently.

Specific to tomato sauce:

Tomato sauce can be acidic enough that you don't care about botulism spores - but that is tricky, since it depends on the variety of tomatoes used, how well the acid will penetrate other ingredients in the sauce, and other factors. In this case, research a recipe that uses that technique and which is currently recognized by food safety authorities to be safe.

It is not safe to can anything containing meat without pressure-canning. My work-around is to use shredded zucchini to add protein to my sauce, and then to add ascorbic acid (vitamin C) powder — or, more rarely, citric acid if ascorbic isn't available — to acidify the tomato base, making heat-canning a safe option.

This calls for a more refined answer then yes or no. (and this will no doubt generate some minus votes...but read me out). At the end of this answer you will find a YES, is you use your sauce reheated.

How would you like to answer "is it safe to cross a road?". There is a risk of harm, so: no?

That is the correct answer, but IMHO, not a usefull one.Nothing is 100 percent safe. You want to know the risks. I have been wrestling with this myself, also because I like to make garlic oil by combining cloves of garlic and oil. Anearobic, alkalic, so ideal for botulism.

These are difficult number to estimate of course, but the incidence of botulism is minute:

In the United States, during 1990–2000, the median number of foodborne botulism cases per year was 23 (range, 17–43 cases) Most cases are sporadic (i.e., they are not part of outbreaks); outbreaks are typically small, involving 2 or 3 persons (see link below).

Now, supposing the ALL these cases come from home canning tomato sauce, and estimating how many americans home can tomatoes, you have an upper range estimate of the risk. I let you do you own estimates. However, the risk is minute, less than 1 in a million is my guess. Your choice.

But there is an even better and hopeful answer. YES, you sauce is perfectly safe to use, if you USE it, that is, reheat it up to minimum 85 degrees. That kills all the TOXINS, not the spores. But spores you will eat when you eat raw food as well, your body can normally easily handle that:

In contrast with the spore, botulinum toxins are temperature sensitive, and all toxins are inactivated by heating to 85°C for 5 min [17]. Conditions in the normal human intestine are not conducive to germination and vegetation of C. botulinum. C. botulinum spores are routinely ingested and excreted by humans without germination, toxin production, or any harm to the person through whom they pass. The exceptions are the small number of infants who develop infant botulism and the handful of adults who develop adult toxemic infectious botulism.

You see, that is why I dont think answers like "NO, because the rules say no and rules are rules" are wrong. Answers like this would deprive you of your sauce thoughtlessly. With my answer you can decide for yourself. Hope this helps.

http://cid.oxfordjournals.org/content/41/8/1167.full